This invention relates generally to the field of enzymatic conversion of biomass to monomeric sugars and particularly to mixing the biomass with enzymes to promote hydrolysis.
Biomass feedstock may be solely lignocellulosic material or a mixture of lignocellulosic and other materials. Polysaccharide biomass is typically a mixture of starch and lignocellulosic materials. The starch may be contained in grains or a refined starch added as feedstock to form the biomass. The biomass feedstock may also include polymers and other materials.
Enzymes, such as cellulose, are mixed with the biomass to promote hydrolysis. Mixing ensures that the enzymes continually and repeatedly move into contact with chemical reaction sites in the biomass. In addition or in place of enzymes other cellulose degrading organisms and biocatalysts, such as thermophilic bacterium or yeast, may be added to the biomass to promote hydrolysis or other degradation of the biomass.
The different feedstock materials and enzymes (or other degrading materials) are mixed together to form the biomass mixture. The biomass mixture may have characteristics similar to a high matter content powder. Liquid may also be added to the biomass mixture to form a high liquid slurry. Liquid is added to liquefy biomass solids and generate a uniform biomass emulsion formed of feedstock and liquids which have significant differences in their characteristics.
Mixers, constant stir reactors and other such mixing or agitation devices may be used to mix and liquefy the feedstock and enzymes to form the biomass mixture. These devices conventionally are cylindrical vessels arranged vertically and having mechanical mixing devices, such as stirrers having radial arms and blades. These mixing devices generally rotate about a vertical shaft and move through the biomass. The period of mixing needed for the biomass mixture depends on the feedstocks used to form the biomass.
Enzymatic liquefaction of lignocellulosic biomass may require several hours of mixing. This mixing process reduces the viscosity of the biomass as the biomass converts from a generally solids composition to a liquefied slurry. Biomass pretreated for enzymatic conversion to monomeric sugars typically starts the mixing process having a fibrous or mud-like consistency. The enzymes added to the biomass typically have a relatively low concentration with respect to the biomass. The biomass and enzyme mixture tends to be highly viscous as it enters a mixing and pretreatment reactor system, which include one or more hydrolysis reactor vessels.
Due to the high viscosity of the biomass entering the hydrolysis reactor vessel, a large force (torque) is needed to turn the mixing devices and properly mix the enzymes with the biomass. The mixing speed of the mixing arms and other mixer components in the mixing chamber is typically below 300 revolutions per minute (rpm). The required mixing force traditionally limits the size of the mixing vessels. The conventional mixing devices tend to be small diameter vessels because the torque needed to rotate the mixing arms increases exponentially with the radial length of the arms. Due to the high viscosity of the biomass, the radial length of the arms is traditionally been short so that the can be moved arms through the biomass. Similarly, the motors that turn the mixing arms have maximum power limitations that constrain the maximum length of the mixing arms. Due to the constraints of the motor and the mechanical strength of the mixing components, the vessels for mixing the highly viscous pre-treated biomass have conventionally been small and narrow.
Further, the mixing vessels for enzymatic liquefaction of lignocellulosic biomass have traditionally been operated in a batch mode rather than a continuous mode. Batch mode is often better suited to situations were several smaller mixing vessels feed a larger downstream vessel, such as a digester or other reactor vessel.
Recirculation of liquefied material to dilute the incoming pretreated biomass has been proposed to decrease the viscosity, and improve the mixing. Recirculation has a disadvantage in that additional mixing volume is required to achieve the desired retention time in the vessel. Batch processing adds volume to the system, as time has to be provided to fill and empty the vessel.
There is a need for large mixing vessels capable of mixing highly viscous biomass with enzymes. These vessels would preferably be continuous flow vessels in which biomass flows continuously in, through and out of the vessel. A large vessel would provide efficient, high flow capacity for mixing biomass and enzymes.